1. Field of the Invention
The present invention relates to a mold clamping force correction method for a mold clamping apparatus which detects variation in the mold clamping force during production and corrects the mold clamping force on the basis of the variation.
2. Description of the Relevant Art
Japanese Patent No. H6-61806 (1994) discloses a toggle-type mold clamping apparatus for clamping a mold provided in an injection molding machine. As disclosed, the toggle-type mold clamping apparatus connects a movable platen supporting a movable mold and a crosshead advanced and retracted by a drive unit with a toggle link mechanism, has the function of increasing the pressing force of the crosshead and transmitting it to the movable platen, generating a predetermined mold clamping force based on the extension of the tie bars in a state in which the toggle link mechanism is almost fully extended. In the mold closing operation, high-velocity mold closure is usually performed from a mold open position and, if the mold reaches a low-velocity low-pressure changeover position, the operation mode is switched to low-velocity low-pressure mold closing. The low-velocity low-pressure mold closure becomes the mold protection zone, from which a molded product that is not properly ejected is detected as a foreign object. If the mold reaches a preset high-pressure changeover position, the operation mode is switched to perform high-pressure mold clamping.
The toggle-type mold clamping apparatus differs from a direct-pressure type clamping apparatus by the mold clamping principle, and the mold clamping force greatly varies even when the mold and tie bars expand and contract only slightly due to foreign disturbance factors, such as the heating temperature of the mold and the outside air temperature, etc. In particular, it causes significant problems, such as quality deterioration, etc. when forming a precise molded product. More specifically, the mold clamping force increases due to the thermal expansion of the mold after heating, even when a regular mold clamping force (target value) is established before heating the mold. Heat is then transmitted from the mold to the tie bars, and the tie bars thermally expand to slowly reduce the mold clamping force when heating the mold or thereafter. Moreover, the thermal expansion of mold becomes a primary factor for increasing the mold clamping force and the thermal expansion of tie bars become a primary factor for reducing the mold clamping force.
Thus, foreign disturbance factors, such as the heating temperature of the mold and the outside air temperature, etc., become significant primary factors to maintaining the mold clamping force correctly in the toggle-type mold clamping apparatus. An injection molding machine in which the variation of the mold clamping force is suppressed by having a toggle-type mold clamping apparatus, which is arranged between a movable platen and mounted with a movable side mold and a rear platen advanced/retracted by a servo motor for clamping the movable platen, and provided with a mold clamping force adjustment means for moving the position of rear platen to adjust the mold clamping force, acquiring a mold clamping force used as a standard, and more specifically, a peak current value used as a standard proportional to the mold clamping force and adjusting the position of rear platen by the mold clamping force adjustment means in accordance with the difference between the peak current value measured for each predetermined cycle in molding cycles thereafter and the peak current value used as standard, is also disclosed in Japanese Laid-Open Patent Application No. 2004-122579.
However, the conventional mold clamping force correction method (for injection molding machine) had the following problems.
First, the peak current value decreases in a small-size mold of small load torque and the peak current value increases in a large-size mold of a large load torque, particularly, an accurate adjustment is difficult to make due to the reduction of sensitivity in a small-size mold and the peak current value (absolute value) greatly differs depending upon the size of mold, the method is poor in the accuracy of mold clamping force adjustment and versatility in use, making matching adjustment necessary for each mold, etc.
Secondly, the peak current value and the mold clamping force are frequently not in proportion depending on the mold (injection molding machine), making the method difficult to use. In an actual injection molding machine, the mold clamping control is also accomplished by torque limitation so that the load torque does not exceed a provided magnitude from the view-point of mold protection more than a few cases, as shown in FIG. 5 and FIG. 6, and it is also possible to use a spring built-in mold C in which springs Bs lie between a movable mold Cm and a stationary mold Cc in several cases, where the peak current value and the mold clamping force are not proportional and the method is deficient in flexibility and reliability.